System and method for protection during inverter shutdown in distributed power installations

ABSTRACT

A protection method in a distributed power system including of DC power sources and multiple power modules which include inputs coupled to the DC power sources. The power modules include outputs coupled in series with one or more other power modules to form a serial string. An inverter is coupled to the serial string. The inverter converts power input from the string and produces output power. When the inverter stops production of the output power, each of the power modules is shut down and thereby the power input to the inverter is ceased.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of co-pending U.S. application Ser. No. 11/950,271 filed Dec. 4, 2007 by the present inventors, the disclosure of which is included herein by reference for all purposes as if entirely set forth herein.

The present application claims priority benefits from U.S. provisional application 60/978,764 filed Oct. 10, 2007 by the present inventors, the entire disclosure of which is incorporated herein by reference.

FIELD AND BACKGROUND

The present invention relates to anti-islanding in a distributed power system and, more particularly, system and method for protection of photovoltaic distributed power equipment and personnel during anti-islanding.

Utility networks provide an electrical power system to utility customers. The distribution of electric power from utility companies to customers utilizes a network of utility lines connected in a grid-like fashion, referred to as an electrical grid. The electrical grid may consist of many independent energy sources energizing the grid in addition to utility companies energizing the grid, with each independent energy source being referred to as a distributed power (DP) generation system. The modern utility network includes the utility power source, consumer loads, and the distributed power generation systems which also supply electrical power to the network. The number and types of distributed power generation systems is growing rapidly and can include photovoltaics, wind, hydro, fuel cells, storage systems such as battery, super-conducting flywheel, and capacitor types, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines. These distributed power generation systems are connected to the utility network such that they operate in parallel with the utility power sources.

One common problem faced by modern utility networks is the occurrence of islanding. Islanding is the condition where a distributed power generation system is severed from the utility network, but continues to supply power to portions of the utility network after the utility power supply is disconnected from those portions of the network. All photovoltaic systems must have anti islanding detection in order to comply with safety regulations. Otherwise the photovoltaic installation may shock or electrocute repairmen after the grid is shut down from the photovoltaic installation generating power as an island downstream. The island condition complicates the orderly reconnection of the utility network and poses a hazard also to equipment. Thus, it is important for an island condition to be detected and eliminated.

Several techniques have been proposed to guard against islanding. For example, one method involves the monitoring of auxiliary contacts on all circuit breakers of the utility system between its main source of generation and DP systems. The auxiliary contacts are monitored for a change of state which represents an open circuit breaker on the utility source. The utility circuit breaker is typically monitored and tripped by external protective relays. When a loss of utility is detected by the change in state of the auxiliary contact of a circuit breaker, a transferred trip scheme is employed to open the interconnection between the utility and the distributed power system. A transferred trip scheme uses the auxiliary contacts of the utility source being monitored. The auxiliary contacts are connected in parallel with other devices which can trigger the trip of the local interconnection breaker. When the auxiliary contacts change state, a trip is induced on the local interconnection breaker. This prevents an island condition from occurring. The drawback of such a method is that often the point of utility isolation (the point at which the utility circuit breaker opens) is of such a distance from the local distributed power system that running a contact status signal back to the local distributed power system control system is not practical.

Anti-islanding schemes presently used or proposed include passive schemes and active schemes. Passive schemes are based on local monitoring of the grid signals, such as under or over voltage, under or over frequency, rate of change of frequency, phase jump, or system harmonics, for example. Active schemes are based on active signal injection with monitoring of the resulting grid signals, such as impedance measurement for example, or active signal injection with active controls, such as active frequency shifting or active voltage shifting for example. With active schemes, some distortion may occur in the output current waveform, thereby resulting in a tradeoff between islanding detection time and waveform distortion, with faster detection typically resulting in higher total harmonic distortion.

A conventional installation of a solar distributed power system 10, including multiple solar panels 101, is illustrated in FIG. 1. Since the voltage provided by each individual solar panel 101 is low, several panels 101 are connected in series to form a string 103 of panels 101. For a large installation, when higher current is required, several strings 103 may be connected in parallel to form overall system 10. The interconnected solar panels 101 are mounted outdoors, and connected to a maximum power point tracking (MPPT) module 107 and then to an inverter 104. MPPT 107 is typically implemented as part of inverter 104 as shown in FIG. 1. The harvested power from DC sources 101 is delivered to inverter 104, which converts the direct-current (DC) into alternating-current (AC) having a desired voltage and frequency, which is usually 110V or 220V at 60 Hz, or 220V at 50 Hz. The AC current from inverter 104 may then be used for operating electric appliances or fed to the power grid.

As noted above, each solar panel 101 supplies relatively very low voltage and current. A problem facing the solar array designer is to produce a standard AC current at 120V or 220V root-mean-square (RMS) from a combination of the low voltages of the solar panels. The delivery of high power from a low voltage requires very high currents, which cause large conduction losses on the order of the second power of the current i². Furthermore, a power inverter, such as inverter 104, which is used to convert DC current to AC current, is most efficient when its input voltage is slightly higher than its output RMS voltage multiplied by the square root of 2. Hence, in many applications, the power sources, such as solar panels 101, are combined in order to reach the correct voltage or current. A large number of panels 101 are connected into a string 103 and strings 103 are connected in parallel to power inverter 104. Panels 101 are connected in series in order to reach the minimal voltage required for inverter 104. Multiple strings 103 are connected in parallel into an array to supply higher current, so as to enable higher power output.

FIG. 1B illustrates one serial string 103 of DC sources, e.g., solar panels 101 a-101 d, connected to MPPT circuit 107 and inverter 104. The current versus voltage (IV) characteristics is plotted (110 a-110 d) to the left of each DC source 101. For each DC power source 101, the current decreases as the output voltage increases. At some voltage value, the current goes to zero, and in some applications the voltage value may assume a negative value, meaning that the source becomes a sink. Bypass diodes (not shown) are used to prevent the source from becoming a sink. The power output of each source 101, which is equal to the product of current and voltage (P=i*V), varies depending on the voltage drawn from the source. At a certain current and voltage, close to the falling off point of the current, the power reaches its maximum. It is desirable to operate a power generating cell at this maximum power point (MPP). The purpose of the MPPT is to find this point and operate the system at this point so as to draw the maximum power from the sources.

In a typical, conventional solar panel array, different algorithms and techniques are used to optimize the integrated power output of system 10 using MPPT module 107. MPPT module 107 receives the current extracted from all of solar panels 101 together and tracks the maximum power point for this current to provide the maximum average power such that if more current is extracted, the average voltage from the panels starts to drop, thus lowering the harvested power. MPPT module 107 maintains a current that yields the maximum average power from system 10.

However, since power sources 101 a-101 d are connected in series to single MPPT 107, MPPT 107 selects a maximum power point which is some average of the maximum power points of the individual serially connected sources 101. In practice, it is very likely that MPPT 107 would operate at an I-V point that is optimum for only a few or none of sources 101. In the example of FIG. 1B, the selected point is the maximum power point for source 101 b, but is off the maximum power point for sources 101 a, 101 c and 101 d. Consequently, the arrangement is not operated at best achievable efficiency.

The present applicant has disclosed in co-pending U.S. application Ser. No. 11/950,271 entitled “Distributed Power Harvesting Systems Using DC Power Sources”, the use of an electrical power converter, e.g. DC-to-DC converter, coupled to the output of each power source, e.g. photovoltaic panel. The electrical power converter converts input power to output power by monitoring and controlling the input power at a maximum power level. This system may be used also to address the anti-islanding issue.

The term “leakage” as used herein refers to electrical power which is radiated or conducted into an electrical signal line typically at low levels and typically because of insufficient isolation.

SUMMARY

The following summary of the invention is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below.

According to an aspect of the present invention, there is provided in a distributed power system multiple DC power sources and multiple power modules which include inputs coupled respectively to the DC power sources. The power modules each include outputs coupled in series to form a serial string. An inverter is coupled to the serial string. The inverter converts power input from the string and produces output power. A protection mechanism in the power modules shuts down the power modules and ceases the power input to the inverter when the inverter stops producing the output power. Typically, the inverter is connected to the electrical grid. A monitoring mechanism is attached to the electrical grid which monitors one or more electrical parameters of the electrical grid. A shutdown mechanism is attached to the monitoring mechanism which when one or more of the electrical parameters is out of predetermined specification, the inverter stops the production of the output power or disconnects from the grid. A switch is preferably disposed between the serial string and the inverter. The switch is activated by the shutdown mechanism and the protection mechanism senses a change in current flowing through the serial string when the switch is activated. When the switch is connected serially with the serial string, the protection mechanism senses that current less than a previously specified minimal threshold current in the serial string; or when the switch is connected in parallel with the serial string the protection mechanism senses a current greater than a previously specified maximal threshold current in the string. Alternatively a signal-providing mechanism is attached to the inverter which provides a signal based on the shutdown mechanism. Multiple receivers are attached respectively to the power modules. The receivers receive the signal and multiple enabling mechanisms, which are attached respectively to the receivers, enable the respective power modules to supply the input power to the inverter based on the presence of the signal or absence thereof. When the signal is a keep-alive signal, the enabling mechanisms enable the respective power modules to supply the input power to the inverter based on the presence of the keep-alive signal. When the signal is a shut-down signal, the enabling mechanism disables the respective power modules and stops supply of the input power to the inverter based on the presence of the shut-down signal. The signal in the serial string is optionally from the electrical grid and detected at the frequency of the electrical grid or detected at a higher frequency up converted from the frequency of the electrical grid. The signal in the serial string is optionally from the inverter or the output power therefrom, and detected at a switching frequency of the inverter. The signal is optionally superimposed on the power input to the inverter from the serial string. The signal may be wirelessly transmitted by the signal-providing mechanism, and the receiver in each of the power modules, receives the wirelessly transmitted signal.

According to another aspect of the present invention, there is provided a protection method in a distributed power system including DC power sources and multiple power modules each of which include inputs coupled to the DC power sources. The power modules each include outputs coupled in series to form a serial string. An inverter is coupled to the serial string. The inverter converts power input from the string and produces output power. When the inverter stops production of the output power, each of the power modules is shut down and thereby the power input to the inverter is ceased. When the inverter is connected to and supplies the output power to the electrical grid, one or more electrical parameters of the grid are monitored. When the one or more electrical parameters of the grid are out of a predetermined specification, the inverter is shut down and thereby production of the output power is stopped or the inverter is disconnected from the grid. When the inverter is shut down, a switch disposed between the serial string and the inverter is activated. When the switch is activated a change in current flowing through the serial string is sensed. Alternatively a signal is provided based on the shutdown mechanism. Multiple receivers are attached respectively to the power modules. The receivers receive the signals which enable the respective power modules to supply the input power to the inverter based on the presence of the signal or absence thereof. When the signal is a keep-alive signal, the respective power modules supply the input power to the inverter based on the presence of the keep-alive signal. When the signal is a shut-down signal, the respective power modules stop supply of the input power to the inverter based on the presence of the shut-down signal. The signal may be based on current in the serial string from the electrical grid and detected at the frequency of the electrical grid or detected at a higher frequency up converted from the frequency of the electrical grid. The signal in the serial string is optionally from the inverter or the output power therefrom, and detected at a switching frequency of the inverter. The signal is optionally actively superimposed on the power input to the inverter from the serial string. The signal may be wirelessly transmitted, and the receiver in each of the power modules, receives the wirelessly transmitted signal.

The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate various features of the illustrated embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not necessarily drawn to scale.

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 illustrates a conventional power harvesting system using photovoltaic panels as DC power sources;

FIG. 1B illustrates current versus voltage characteristic curves for one serial string the DC power sources of FIG. 1;

FIG. 2 illustrates a distributed power harvesting circuit, previously disclosed in co-pending U.S. application Ser. No. 11/950,271;

FIG. 3 illustrates an exemplary DC-to-DC converter, previously disclosed in co-pending U.S. application Ser. No. 11/950,271;

FIGS. 4 and 4A illustrate a system for protection during an islanding condition, in accordance with aspects of the present invention;

FIGS. 4B and 4C illustrate in more detail the system of FIGS. 4 and 4A;

FIG. 4D illustrates a method, according to an aspect of the present invention using the system of FIGS. 4 and 4A.

FIGS. 5 and 5A, illustrate a system for protection during an islanding condition in accordance with other aspects of the present invention;

FIG. 5B illustrates an example wherein a system according to an embodiment of the invention is applied as a retrofit to a prior art system, such as the system of FIG. 1.

FIGS. 6, 6A and 6B illustrate a system for protection during an islanding condition, according to still other aspects of the present invention; and

FIGS. 7 and 7A, illustrate a system for protection during an islanding condition, according to yet other aspects of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.

It should be noted, that although the discussion herein relates primarily to anti-islanding in photovoltaic systems and more particularly to those systems previously disclosed in U.S. application Ser. No. 11/950,271, the present invention may, by non-limiting example, alternatively be configured as well using conventional photovoltaic distributed power systems and other distributed power systems including (but not limited to) wind turbines, hydroturbines, fuel cells, storage systems such as battery, super-conducting flywheel, and capacitors, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines.

By way of introduction, it is important to note that aspects of the present invention have important safety benefits. While installing or performing maintenance on photovoltaic systems according to certain aspects of the present invention, installers are protected from danger of shock or electrocution since systems according to embodiments of the present invention do not output high voltage such as when solar panels are exposed to sunlight. Similarly, firefighters, even after they shut down the main electrical switch to a burning building can safely break into the burning building or hose the roof of the building with water without fear of high voltage DC conduction through the water, since high voltage direct current feeding the inverter is safely turned off.

Before explaining embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

Referring now to the drawings, FIG. 2 illustrates a distributed power harvesting circuit 20, previously disclosed in U.S. application Ser. No. 11/950,271. Circuit 20 enables connection of multiple distributed power sources, for example solar panels 201 a-201 d, to a single power supply. Series string 203 of solar panels 201 may be coupled to an inverter 204 or multiple connected strings 203 of solar panels 201 may be connected to a single inverter 204. In configuration 20, each solar panel 201 a-201 d is connected individually to a separate power conditioner, here a converter circuit or a module 205 a-205 d. Each solar panel 201 together with its associated power converter circuit 205 forms a power generating element 222. (Only one such power generating element 222 is marked in FIG. 2.) Each converter 205 a-205 d adapts optimally to the power characteristics of the connected solar panel 201 a-201 d and transfers the power efficiently from input to output of converter 205. Converters 205 a-205 d are typically microprocessor controlled switching converters, e.g. buck converters, boost converters, buck/boost converters, flyback or forward converters, etc. The converters 205 a-205 d may also contain a number of component converters, for example a serial connection of a buck and a boost converter. Each converter 205 a-205 d includes a control loop 221, e.g. MPPT loop that receives a feedback signal, not from the converter's output current or voltage, but rather from the converter's input coming from solar panel 201. The MPPT loop of converter 205 locks the input voltage and current from each solar panel 201 a-201 d at its optimal power point, by varying one or more duty cycles of the switching conversion typically by pulse width modulation (PWM) in such a way that maximum power is extracted from each attached panel 201 a-201 d. The controller of converter 205 dynamically tracks the maximum power point at the converter input. Feedback loop 221 is closed on the input power in order to track maximum input power rather than closing a feedback loop on the output voltage as performed by conventional DC-to-DC voltage converters.

As a result of having a separate MPPT circuit in each converter 205 a-205 d, and consequently for each solar panel 201 a-201 d, each string 203 may have a different number or different specification, size and/or model of panels 201 a-201 d connected in series. System 20 of FIG. 2 continuously performs MPPT on the output of each solar panel 201 a-201 d to react to changes in temperature, solar radiance, shading or other performance factors that effect one or more of solar panels 201 a-201 d. As a result, the MPPT circuit within the converters 205 a-205 d harvests the maximum possible power from each panel 201 a-201 d and transfers this power as output regardless of the parameters effecting other solar panels 201 a-201 d.

The outputs of converters 201 a-201 d are series connected into a single DC output that forms the input to inverter 204. Inverter 204 converts the series connected DC output of converters 201 a-201 d into an AC power supply. Inverter 204, may be set to regulate the voltage at the input of inverter 204. In this example, an independent control loop 220 holds the voltage input to inverter 204 at a set value, say 400 volts. The current at the input of inverter 204 is typically fixed by the power available and generated by photovoltaic panels 201.

In order to legally be allowed to connect to the grid in each country, inverter 104, 204 is preferably designed to comply with local electrical regulations. Electrical regulations typically dictate, among other things, the minimal and maximal voltages of the grid e.g. 220-260 root mean squares voltage V, and a range of permitted frequency, e.g 45-55 Hz. Whenever the grid deviates from allowed values inverter 104, 204 is required to disconnect from the grid. Disconnection from the grid is typically performed using software controlling inverter 104, 204 and control circuitry which constantly monitors grid parameters, e.g. voltage, frequency.

In system 10, solar panels 101 are directly connected (e.g. in series-parallel) to inverter 104. When an islanding condition is detected, inverter 104 is disconnected from the grid. Hence, inverter 104 stops drawing current and therefore panels 101 output a relatively high open circuit voltage typically 25% higher than the normal operating voltage. An open circuit voltage 25% higher than nominal working voltage is typically safe, (less than the allowed 600VDC in the USA and 1000VDC in Europe) which are typical ratings for inverters 104 designed to be able to handle the higher open circuit voltage.

In system 20, there are power converters 205 which “push” power to the output of converters 205. Under an islanding condition which has been detected by inverter 204, inverter 204 is shut down and current is not flowing between converters 205 and inverter 204. Consequently, in system 20, the open circuit voltage at the input to inverter 204, reaches dangerous voltages, higher than the open circuit maximum voltage ratings of inverters 104, 204.

Reference is now made to FIG. 3 which illustrates an exemplary DC-to-DC converter 205 previously disclosed in co-pending U.S. application Ser. No. 11/950,271. DC-to-DC converters are used to either step down or step up a DC voltage input to a higher or a lower DC voltage output, depending on the requirements of the output circuit. However, in the embodiment of FIG. 3 the DC-DC converter 205 is used as a power converter, i.e., transferring the input power to output power, the input voltage varying according to the MPPT at the input, while the output current is dictated by the constant input voltage to inverter 104, 204. That is, the input voltage and current may vary at any time and the output voltage and current may vary at any time, depending on the operating condition of DC power sources 201.

Converter 205 is connected to a corresponding DC power source 201 at input terminals 314 and 316. The converted power of the DC power source 201 is output to the circuit through output terminals 310, 312. Between the input terminals 314, 316 and the output terminals 310, 312, the converter circuit includes input and output capacitors 320, 340, backflow prevention diodes 322, 342 and a power conversion circuit including a controller 306 and an inductor 308.

Diode 342 is in series with output 312 with a polarity such that current does not backflow into the converter 205. Diode 322 is coupled between the positive output lead 312 through inductor 308 which acts a short for DC current and the negative input lead 314 with such polarity to prevent a current from the output 312 to backflow into solar panel 201.

A potential difference exists between wires 314 and 316 due to the electron-hole pairs produced in the solar cells of panel 201. Converter 205 maintains maximum power output by extracting current from the solar panel 201 at its peak power point by continuously monitoring the current and voltage provided by panel 201 and using a maximum power point tracking algorithm. Controller 306 includes an MPPT circuit or algorithm for performing the peak power tracking. Peak power tracking and pulse width modulation (PWM) are performed together to achieve the desired input voltage and current. The MPPT in controller 306 may be any conventional MPPT, such as, e.g., perturb and observe (P&O), incremental conductance, etc. However, notably the MPPT is performed on panel 201 directly, i.e., at the input to converter 205, rather than at the output of converter 205. The generated power is then transferred to the output terminals 310 and 312. The outputs of multiple converters 205 may be connected in series, such that the positive lead 312 of one converter 205 is connected to the negative lead 310 of the next converter 205.

In FIG. 3, converter 205 is shown as a buck plus boost converter. The term “buck plus boost” as used herein is a buck converter directly followed by a boost converter as shown in FIG. 3, which may also appear in the literature as “cascaded buck-boost converter”. If the voltage is to be lowered, the boost portion is substantially shorted. If the voltage is to be raised, the buck portion is substantially shorted. The term “buck plus boost” differs from buck/boost topology which is a classic topology that may be used when voltage is to be raised or lowered, and sometimes appears in the literature as “cascaded buck-boost”. The efficiency of “buck/boost” topology is inherently lower then a buck or a boost. Additionally, for given requirements, a buck-boost converter will need bigger passive components then a buck plus boost converter in order to function. Therefore, the buck plus boost topology of FIG. 3 has a higher efficiency than the buck/boost topology. However, the circuit of FIG. 3 continuously decides whether it is bucking or boosting. In some situations when the desired output voltage is similar to the input voltage, then both the buck and boost portions may be operational.

The controller 306 may include a pulse width modulator, PWM, or a digital pulse width modulator, DPWM, to be used with the buck and boost converter circuits. Controller 306 controls both the buck converter and the boost converter and determines whether a buck or a boost operation is to be performed. In some circumstances both the buck and boost portions may operate together. That is, the input voltage and current are selected independently of the selection of output current and voltage. Moreover, the selection of either input or output values may change at any given moment depending on the operation of the DC power sources. Therefore, in the embodiment of FIG. 3, converter 205 is constructed so that at any given time a selected value of input voltage and current may be up converted or down converted depending on the output requirement.

In one implementation, an integrated circuit (IC) 304 may be used that incorporates some of the functionality of converter 205. IC 304 is optionally a single ASIC able to withstand harsh temperature extremes present in outdoor solar installations. ASIC 304 may be designed for a high mean time between failures (MTBF) of more than 25 years. However, a discrete solution using multiple integrated circuits may also be used in a similar manner. In the exemplary embodiment shown in FIG. 3, the buck plus boost portion of the converter 305 is implemented as the IC 304. Practical considerations may lead to other segmentations of the system. For example, in one aspect of the invention, the IC 304 may include two ICs, one analog IC which handles the high currents and voltages in the system, and one simple low-voltage digital IC which includes the control logic. The analog IC may be implemented using power FETs which may alternatively be implemented in discrete components, FET drivers, A/Ds, and the like. The digital IC may form controller 306.

In the exemplary circuit 205 shown, the buck converter includes input capacitor 320, transistors 328 and 330, diode 322 positioned in parallel to transistor 328, and inductor 308. Transistors 328, 330 each have a parasitic body diode 324, 326. The boost converter includes inductor 308, which is shared with the buck converter, transistors 348 and 350 a diode 342 positioned in parallel to transistor 350, and output capacitor 340. Transistors 348, 350 each have a parasitic body diode 344, 346.

System 20 includes converters 205 which are connected in series and carry the current from string 203. If a failure in one of the serially connected converters 205 causes an open circuit in failed converter 205, current ceases to flow through the entire string 203 of converters 205, thereby causing system 20 to stop functioning. Aspects of the present invention provide a converter circuit 205 in which electrical components have one or more bypass routes associated with them that carry the current in case of an electrical component failing within one of converters 205. For example, each switching transistor of either the buck or the boost portion of the converter has its own diode bypass. Also, upon failure of inductor 308, the current bypasses the failed inductor 308 through parasitic diodes 344, 346.

Reference is now made to FIG. 4 which illustrates a system 40 for protection during an islanding condition, in accordance with embodiments of the present invention. For simplicity, a single string 423 is shown of distributed power sources, e.g solar panels 201 a-201 d connected to respective power converters 405 a-d. Serial string 423 is input to inverter 404 through wires 412 and 410. The output of inverter 404 is connected to and supplies electrical power to the electrical grid. Inverter 404, typically includes a monitoring, and detection mechanism 401 which monitors one or more parameters of the electrical grid such as voltage and/or frequency. If one or more of the grid parameters is out of specification indicating an islanding condition, monitoring and detection mechanism 401 typically causes inverter 404 to be shut down or inverter 404 is disconnected from the grid so that output power is no longer supplied by inverter 404 to the grid. At the same time, a signal 414 is transmitted to a switch mechanism 403 which may be located at the input of inverter 404 before input capacitor 408. Switch mechanism 403 is optionally packaged with inverter 404 or may be integrated with inverter 404 and packaged separately. In this example, signal 414 activates switch mechanism 403 so that when switch 403 is activated, the current flowing through serial string 423 and wires 410, 412 varies abruptly.

Reference is now also made to FIG. 4A which illustrates in more detail converter 405. Converter 405 is equipped with a current sensing mechanism 407 which upon sensing a variation in current through serial string 423 signals controller 306 to shut down and stop converting power. Typically, current sensing mechanism 407 includes an analog/digital converter which continuously feeds data to controller 306. Controller 306 detects a shutdown in current and decides to shut down the converters 405 accordingly.

Reference is now also made to FIGS. 4B and 4C which illustrate schematically switch mechanism 403 in more detail. FIG. 4B illustrates switch mechanism 403 in a serial configuration in which switch 403 is connected in series with the serial string 423 and FIG. 4C illustrates a parallel configuration in which switch 403 is connected in parallel with serial string 423. In the serial configuration (FIG. 4B) switch 403 is closed during normal operation of inverter 404. When an island condition is detected, serial switch 403 opens during shut down of inverter 404. Current sensing mechanism 407 upon sensing zero current signals controller 306 that output current is less than a previously specified minimum value and controller 306 shuts down power conversion in converter 405. In the parallel configuration (FIG. 4C), switch 403 is open during normal operation of inverter 404. When an island condition is detected, parallel switch 403 closes during shut down of inverter 404. With all the current of serial string 423 flowing through the switch 403 at minimal load, the current increases to above a previously specified maximum current. Current sensing mechanism 407 upon sensing a current maximum signals controller 306 that output current is above maximal previously specified value and controller 306 shuts down power conversion. Switch mechanism 403 in different embodiments may be embodied by a mechanical switch or a solid state switch with current and voltage ratings appropriate to the present application. Switch mechanism 403 is preferably selected by one skilled in the art of power electronics so that arcing across its open terminals is avoided while practicing some embodiments of the present invention.

Reference is now made FIG. 4D which illustrates a method, according to an embodiment of the present invention. In decision block 450, output power from inverter 104, 204 is constantly monitored. If output power is stopped, power converters 405 are shut down.

Reference is now made to FIG. 5, illustrating a system 50 according to other embodiments of the present invention for protection during an islanding condition. For simplicity, a single string 523 is shown of distributed power sources, e.g solar panels 201 a-201 d connected to respective power converters 505 a-d. Serial string 523 is input to inverter 504 through wires 412 and 410. The output of inverter 504 is connected to and supplies electrical power to the electrical grid. Inverter 504, typically includes a monitoring and detection mechanism 401 which monitors one or more parameters of the electrical grid such as voltage and/or frequency. If one or more of the grid parameters is out of specification indicating an islanding condition, monitoring/detection mechanism 401 typically shuts down inverter 504 or disconnects from the grid, so that output power is no longer supplied by inverter 504 to the grid. During normal operation, a line communications transmitter 503 superimposes a keep-alive signal, for instance between 1 kilohertz to 100 Megahertz on direct current (DC) input lines 410 and 412 attached to serial string 523.

Reference is now also made to FIG. 5A which illustrates converter 505 in more detail. The keep-alive signal is constantly monitored and detected by a line communications receiver 507. Only while receiver 507 senses the keep-alive signal does receiver 507 provide an enable signal to controller 306. When controller 306 doesn't receive an enabling signal from receiver 507, controller 306 shuts down power conversion of converter 505.

Alternatively, instead of a “keep-alive” signal, a stop signal 514 which is first generated by monitoring and detection mechanism 401 when an islanding condition is detected, is transmitted to receiver 507. The stop signal is transmitted over line communications by superimposing a varying (e.g. 10 Khz to 100 Mhz) signal over the power lines of serial string 523. Receiver 507 receives the stop signal and relays the stop signal to controller 306 using, e.g., a single disable bit. Controller 306 on receiving a disable signal, stops converting power to the output of converter 505. Typically, when converters 505 are disabled they go into a bypass mode which allows current from other converters 505 to pass through. Hence, the stop signal may be continued until all power stops being supplied on string 523 by all of converters 505.

It should be noted that one skilled in the art would realize that although in system 50, converters 505 are shown to have feedback loop 221, as in controller 205 of system 20, embodiments of the present invention as illustrated in system 40 using switch mechanism 403 and/or in system 50 using line communications, to the serial string may be applied to and find benefit in other distributed power systems using converters without feedback loops 221 as applied to prior art system 10. Similarly, conventional inverters 104 may be used instead of inverter 504 with communications transmitter 503 added to inverter 104 either by the inverter manufacturer or as a retrofit. For example, FIG. 5B illustrates a system according to an embodiment of the invention applied as a retrofit to a prior art system, such as the system of FIG. 1. In this example, detection mechanism 401 and switch mechanism 403 are installed between the grid and the conventional inverter 104. Of course, detection mechanism 401 and switch mechanism 403 may be incorporated into the inverter, e.g., for original installation, rather than a retrofit. Also, other implementations described herein may be used instead of detection mechanism 401 and switch mechanism 403. Advantages of incorporation of monitoring and detection mechanism 401 and one of switch mechanism 403 or communications transmitter 503 into system 10 is beneficial during installation, maintenance, and firefighting.

Reference in now made to FIG. 6 which illustrates system 60, according to another embodiment of the present invention for protection during an islanding condition. For simplicity, a single string 623 is shown of distributed power sources, e.g solar panels 201 a-201 d connected to respective power converters 605 a-d. Serial string 623 is input to conventional inverter 104 through wires 412 and 410. The output of inverter 104 is connected to and supplies electrical power to the electrical grid. Inverter 104, typically includes a monitoring and detection mechanism 401 which monitors one or more parameters of the electrical grid such as voltage and/or frequency. If one or more of the grid parameters is out of specification indicating an islanding condition, monitoring and detection mechanism 401 typically shuts down inverter 104 so that output power is no longer supplied by inverter 104 to the grid. During normal operation, a 100 Hz (or 120 Hz. in USA) ripple current is detectable between lines 410, 412 and in serial string 623 since capacitors of inverter 104 do not block entirely the alternating current (AC), or the 100/120 Hz is intentionally leaked into serial string 623 through lines 410, 412.

Reference is now also made to FIG. 6A which illustrates converter 605 in more detail. The 100/120 Hz leakage is constantly monitored and detected by a receiver 607. Only while receiver 607 senses the leakage from the grid does receiver 607 provide an enable signal to controller 306. When controller 306 doesn't receive an enabling signal from receiver 607, controller 306 shuts down power conversion of converters 605.

Alternatively or in addition, one or more switching frequencies of inverter 104, typically 16 Khz or 32 KHz. may be detected as leakage or provided intentionally to serial string 623 along lines 412, 410. Receiver 607 is configured to detect the 16/32 KHz inverter switching frequency and provides an enabling signal to controller while inverter 104 is operating.

Reference is now made to FIG. 6B, showing a simplified block diagram according to an embodiment of the present invention for up conversion of 100/120 Hz. into a higher frequency in order to enable faster detection in receiver 607 of leakage from the grid. The 100 Hertz or 100 Hertz signal is AC coupled by capacitor 631 to remove the direct current component in serial string 623 and lines 410 and 412. The 100/120 Hz. signal is optionally amplified and rectified by a full wave rectifier 635 so that a 100 Hz or 120 Hz unipolar DC ripple is achieved. The 100/120 Hz unipolar signal is split. One portion of the 100/120 Hz. unipolar ripple is converted to a square wave, such as in a comparator/digitize circuit 639. A second portion of the 100/120 Hz unipolar ripple undergoes a known phase shift, e.g of 400 Hz. in a phase shifter 633 and output to a second comparator/digitizing circuit 631. The two outputs of two digitizing circuits 639, 631 undergo an exclusive OR in a XOR circuit 633 which outputs a signal at a much higher frequency, e.g. 800 Hz.

Reference is now made to FIG. 7, illustrating a system 70 according to other embodiments of the present invention for protection during an islanding condition. For simplicity, a single string 723 is shown of distributed power sources, e.g solar panels 201 a-201 d connected to respective power converters 705 a-d. Serial string 723 is input to inverter 704 through wires 412 and 410. The output of inverter 704 is connected to and supplies electrical power to the electrical grid. Inverter 704, typically includes a monitoring and detection mechanism 401 which monitors one or more parameters of the electrical grid such as voltage and/or frequency. If one or more of the grid parameters is out of specification indicating an islanding condition, monitoring, and detection mechanism 401 typically shuts down inverter 704 or disconnects inverter 704 from the grid so that output power is no longer supplied by inverter 704 to the grid. During normal operation, a wireless transmitter 703 transmits wirelessly a signal, for instance between 100 Megahertz-10 Gigahertz.

Reference is now also made to FIG. 7A which illustrates converter 705 in more detail. The wireless signal is received and constantly monitored by a wireless receiver 707 Only while receiver 707 senses the wireless signal does receiver 707 provide an enable signal to controller 306. When controller 306 doesn't receive an enabling signal from receiver 707, controller 306 shuts down power conversion of converter 705.

The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Various aspects and/or components of the described embodiments may be used singly or in any combination in the server arts. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 

The invention claimed is:
 1. An apparatus comprising: a power converter configured to convert input power to direct-current output power on a converter output terminal; and a controller circuitry configured to: enable the power converter in response to a presence of an alternating-current signal superimposed on the converter output terminal, and disable the power converter in response to an absence of the alternating-current signal.
 2. The apparatus of claim 1, further comprising an inverter comprising an inverter input terminal and an inverter output terminal, the inverter input terminal being connected to the converter output terminal, wherein the controller circuitry is configured to: enable the power converter in response to the presence of the alternating-current signal at a switching frequency of the inverter, and disable the power converter in response to the absence of the alternating-current signal at the switching frequency of the inverter.
 3. The apparatus of claim 2, wherein the controller circuitry is configured to detect the presence or the absence of the alternating-current signal at a frequency of alternating-current power generated on the inverter output terminal.
 4. The apparatus of claim 1, further comprising a receiver circuit configured to: detect the presence or the absence of the alternating-current signal; and output to the controller circuitry a control signal indicative of the presence or the absence of the alternating-current signal.
 5. The apparatus of claim 1, further comprising a receiver circuit configured to: generate a measurement of the converter output terminal; obtain the alternating-current signal at a first frequency by removing a direct current component of the measurement; up-convert the alternating-current signal at the first frequency to a second alternating-current signal at a second frequency higher than the first frequency; and generate a control signal based on the second alternating-current signal.
 6. The apparatus of claim 1, further comprising an inverter comprising an inverter input terminal and an inverter output terminal, the inverter input terminal being connected to the converter output terminal, the alternating-current signal being indicative of the inverter converting the direct-current output power from the power converter to alternating-current power on the inverter output terminal.
 7. The apparatus of claim 6, further comprising a monitoring circuit configured to: detect an electrical parameter on the inverter output terminal; and shutdown operation of the inverter in response to the electrical parameter being out of a predetermined specification.
 8. The apparatus of claim 6, further comprising a monitoring circuit configured to: detect an islanding condition on the inverter output terminal; and shutdown operation of the inverter in response to detecting the islanding condition.
 9. The apparatus of claim 1, further comprising an inverter comprising an inverter input terminal connected to the converter output terminal, the inverter being configured to convert the direct-current output power from the power converter to alternating-current power on an inverter output terminal, the alternating-current signal being leakage from the inverter output terminal to the inverter input terminal.
 10. The apparatus of claim 9, the alternating-current signal comprising a 100 Hz or 120 Hz ripple current.
 11. The apparatus of claim 1, wherein the power converter comprises a plurality of direct-current-to-direct-current (DC/DC) power converters having DC/DC outputs connected in a serial string to form the converter output terminal.
 12. The apparatus of claim 11, wherein each of the plurality of DC/DC power converters comprises a respective DC/DC input terminal and respective maximum power point tracking circuitry, and wherein each respective maximum power point tracking circuitry is configured to perform maximum power point tracking of a portion of the input power.
 13. The apparatus of claim 1, wherein the power converter comprises a direct-current-to-direct-current (DC/DC) power converter comprising a DC/DC input terminal and maximum power point tracking circuitry configured to perform maximum power point tracking of the input power received on the DC/DC input terminal.
 14. A method comprising: monitoring a presence or an absence of an alternating-current signal superimposed on a converter output terminal of a power converter; enabling, in response to the presence of the alternating-current signal, the power converter to convert input power to direct-current output power on the converter output terminal; and disabling, in response to the absence of the alternating-current signal, the power converter to convert the input power to the direct-current output power on the converter output terminal.
 15. The method of claim 14, wherein the monitoring of the presence or the absence of the alternating-current signal being at a switching frequency of an inverter or a frequency of a power grid.
 16. The method of claim 14, further comprising: generating a measurement of the converter output terminal; obtaining the alternating-current signal at a first frequency by removing a direct current component of the measurement; up-converting the alternating-current signal at the first frequency to a second alternating-current signal at a second frequency higher than the first frequency; generating a control signal based on the second alternating-current signal; and performing the monitoring based on the control signal.
 17. The method of claim 14, further comprising: inverting, with an inverter, the direct-current output power to alternating-current output power to a load, wherein, the alternating-current signal oscillates at a switching frequency of the inverter or a frequency of the load; detecting an islanding condition of the load; and disabling the inverting of the direct-current output power to the alternating-current output power in response to detecting the islanding condition.
 18. The method of claim 14, further comprising performing the conversion by the power converter by: performing direct-current-to-direct-current (DC/DC) power conversion with the power converter of the input power to the direct-current output power; and performing maximum power point tracking of the input power.
 19. The method of claim 14, further comprising performing the conversion by the power converter by: performing direct-current-to-direct-current (DC/DC) power conversion with the power converter of the input power to the direct-current output power, the power converter comprising a plurality of DC/DC power converters having DC/DC outputs connected in a serial string to form the converter output terminal, each DC/DC power converter receiving a respective portion of the input power; and performing maximum power point tracking of each respective portion of the input power independently. 